Edge-state signature in optical absorption of nanographenes: Tight-binding method and time-dependent density functional theory calculations

The optical absorption of triangular nanographenes with well-defined edge structure, that is, zigzag- and armchair-type edges, is studied using the tight-binding method within the Huckel approximation. The absorption spectra of zigzag triangular nanographenes exhibit rich peak structures originating from the excitations associated with the edge states, while those of armchair ones do not. The main feature in the absorption spectra is reproduced by time-dependent density functional theory calculations within a real-time scheme. By varying the size of the nanographenes, we elucidate the role of the edge states in the optical absorption, which becomes conspicuous when the graphene size is on a nanometer scale. Our finding in this paper provides a way to capture the edge-state signature by optical experiments with visible light.